In the prior art, developments have been made of analytical elements for detection of specific biochemical substances present in body fluids and they have been used for clinical tests and others. These are based on utilization of the phenomenon of coloration of a reagent impregnated in the analytical element with a substance to be detected. Recently, use is made widely of analytical elements comprising filter papers, etc. impregnated with reagents.
For example, U.S. Pat. No. 3,050,373 or No. 3,061,523 discloses preparation of an analytical element by impregnating a fibrous porous layer such as a filter paper with a reagent layer, followed by drying. Further, various attempts have been made to avoid interferences in detection with the use of such an element.
For example, as disclosed in Japsnese Patent Publication No. 39558/1975, a filter paper impregnated with a reagent solution and dried is further impregnated with a solution such as of ethyl cellulose and dried to be coated with a semipermeable film, for the purpose of suppressing excessive adhesion of a liquid sample or controlling water retention. Alternatively, as disclosed in Japanese Patent Publication No. 6551/1978, a net of fine mesh such as a fabric or a web is covered on a filter paper impregnated with a reagent for protection from contamination such as by hand contact.
These analytical elements are easy in handling and can give immediate results. However, since uneven coloration of analytical elements may frequently be caused whereby analytical results may be varied, they are inferior in quantitative characteristic. As a consequence, they are useful only for qualitative or semiquantitative analysis. This is because, in preparation of a fibrous porous layer such as filter paper, it is in the first place difficult to obtain a product of uniform structure, and hence no uniform analysis of a reagent is usually possible on impregnation with the reagent. Further, so called chromatographic phenomenon may also occur when using such a fibrous filter paper, whereby excessive ununiform migration of the sample component to be analyzed or the reaction reagent may occur in the reagent layer to bring about localized higher concentrations of such components in the layer.
As improvements over the drawbacks of the analytical test strip of the prior art, which have also improved dramatically the quantitative characteristic thereof, there are known multi-layer analytical elements as disclosed in, for example, Japanese Patent Publication No. 21677/1978 and Japanese Provisional Patent Publication No. 24576/1981.
However, the above analytical elements also suffer from serious deterioration of various reagents contained in the reagent layers during the period of storage, thus giving marked disadvantages with respect to precision and reliability of the analyses. There was caused by deterioration of the activity of peroxidase contained in the analytical element which will exhibit catalytic activity for oxidation of an indicating composition, because the material constituting the analytical element is permeable to an aqueous fluid and therefore greatly influenced by the air and water.
Accordingly, formation of a detectable product which is proportional correctly and without contradiction to the concentration of a component in an aqueous fluid is obstructed.
Further, when an aqueous fluid which is the sample to be tested is applied to the aforesaid analytical element, a large amount of an aqueous fluid per unit area is contained in a reagent layer comprising a hydrophilic colloidal substance, and said element is incubated at a temperature of room temperature or higher. As a result, at the aqueous fluid absorbing portion of the reagent layer in the analytical element, there occurs undesirable swelling or dissolution of the hydrophilic colloidal substance of said layer, whereby no uniform color formation can be expected because of migration of the detectable substance, namely marked increase of the chromatographic phenomenon.
A proposal to improve the above drawback is described in detail in Japanese Provisional Patent Publication No. 50393/1979. That is, storability of a reagent, especially peroxidase is stated to be markedly improved by addition of a water-dispersible polymer comprising a copolymer of anionic monomers, namely a polymer latex to said reagent layer.
However, the above polymer latex is known to interact strongly with gelatin, thereby causing frequently agglomeration during preparation of a coating solution.
As other drawbacks, anionic monomers are difficult incorporate into latex particles and this method has also the disadvantage in preparation to cause agglomeration during preparation of the copolymer.
The present inventors have made extensive studies to overcome the above drawbacks.
The first object of this invention is to provide an analytical element improved in storability of a reagent. The second object of this invention is to provide an analytical element which enables quantitative and simple measurement without requiring any skilled operational procedure.